Multidirectional surveillance marker

Abstract: A marker (10) to be used in article surveillance systems of the type which employ alternating magnetic interrogating fields. The marker comprises a closed loop band of material having high magnetic permeability and low magnetic coercivity; and it has cutouts (12) distributed along its length to form thin strips (10a,10b) of the marker material to increase the effective length to cross section ratio.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to markers or targets for electronic article surveillance and more particularly it concerns a novel magnetically saturable marker characterized by a multidirectional sensitivity. Description of the Related Art United States Patents No. 3,820,103, No. 3,820,104 and No. 3,665,449 describe magnetically saturable markers for use in electromagnetic type surveillance systems. These systems are generally used to reduce theft by shoplifting but they are also used to detect the unauthorized movement of articles in other environments. The markers shown in these patents are elongated strips of magnetically soft material having a high magnetic permeability and a low magnetic coercivity. When exposed to an alternating magnetic field in an interrogation region, for example, the exit from a store, these targets or markers become magnetically saturated in opposite directions during each cycle of the alternating magnetic interrogation field. As a result they disturb the field in a characteristic manner to produce harmonics which can be detected. These patents explain that the targets or markers should have the configuration of an elongated open strip with a high ratio of length to cross-sectional area for maximum sensitivity to the interrogating magnetic fields. When the ratio of length to the square root of cross sectional are is below 150, the magnetization reversal signal amplitude decreases radically and becomes noticeably dependent on orientation of the open strip within the magnetic field. It is also suggested in these patents that the open strip may have one or more major dimensions satisfying this criteria. U.S. Patent No. 3,665,449 mentions the possibility of using a disc shaped marker but indicates that because of the low length to cross-section ratio such markers do not produce very large or distinctive response signals. U.S. Patent No. 4,074,249 describes a crescent shaped target which is less sensitive to orientation than the straight elongated open strip markers. U.S. Patent No. 4,075,618 describes thin elongated markers which have enlarged flux concentrators at the ends thereof to enable the markers to be shortened without reduction in their sensitivity. One problem that characterizes all prior art magnetically saturable markers for electronic article surveillance, is that the shape and size of the available space on the object to be protected by the marker often will not accomodate a thin elongated strip. Compact disc recordings, for example, have only a small ring shaped region araound a center opening that would allow for the placement of a marker. On the other hand, when the strips are shortened to fit into small areas, their sensitivity becomes highly dependent on their orientation relative to the interrogating magnetic field. This is a problem when the markers are used for theft detection because in that case, it is not possible to control their orientation when they are carried through an interrogation zone. To overcome this problem there is known from EP-A2 0 412 721 a magnetic article surveillance marker having a multidirectional sensitivity. This marker comprises two sheets of magnetic material, wherein the first sheet is formed of a ferromagnetic material having high permeability and low coerciv force properties, such as permalloy or the like. The first sheet may be formed to have notches centered along each edge of the sheet. The second sheet of the marker comprises a solid sheet of a magnetizable material and is joined together with the first sheet, and the combination of the two sheets is affixed to articles to be protected. Further, in document US 4,967,184 is disclosed an electronic surveillance marker according to the preamble of claim 1. In particular, this document proposes the use of a ring-shaped surveillance marker which is made from an amorphous ferromagnetic material and adhered to a computer disk such that the ring-shaped surveillance marker surrounds the center opening of the computer disk. The material of the ring-shaped surveillance marker is preferably selected to have low coercivity and a high magnetic permeability. When using the computer disk, the presence of the ring-shaped surveillance marker is detected for security purposes by means of a low strength interrogating sensor field. SUMMARY OF THE INVENTION Starting from the prior art known from US 4,967,184, it is an object of the present invention to provide an electronic surveillance marker having multidirectional sensitivity and also having a higher sensitivity. According to the present invention, this object is achieved by an electronic surveillance marker including the features of claim 1. In particular, the electronic surveillance marker of the present invention comprises a band in the shape of a closed loop, for example a ring-shaped band, wherein a plurality of cutout regions are formed in the band and are spaced along the length of the band. Furthermore, the cutout regions are completely formed within the band, i.e. the cutout regions are enclosed between the inner and outer edges of the band, so that each cutout region forms a pair of narrowed segments on opposite sides of each cutout region. As described in greater detail hereinbelow, it has been found that an electronic surveillance marker having the features of claim 1 produces good responses to a magnetic interrogating field in an electronic surveillance system, with reduced dependence on orientation relative to the interrogating field. These good responses are due to the fact that the cutout regions produce very narrow lenghts on opposite sides of each cutout region, wherein the ratio of length to cross-sectional area of these narrow lengths is relatively high. Furthermore, between the cutout regions material regions are formed which provide large masses fot deactivation purposes. As the marker is formed in the shape of a closed loop and the narrow segments formed on opposite sides of each cutout region extend in different directions at different locations along the marker, the orientation sensitivity of the marker relative to the directions of the interrogating magnetic field is minimized. Further, the forming of the cutout regions within the material of the closed loop marker results in two narrowed regions on opposite sides of each cutout region, wherein these narrowed regions are formed at flux concentrating regions of the marker and, therefore, the marker is made more sensitive. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of a marker according to the present invention; Fig 2 is a plan view showing a compact disc to which the marker of Fig. 1 is attached; Fig. 3 is a plan view of an alternate form of marker according to the present invention; and Fig. 4 is a plan view of a further alternate form of marker according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An electronic article surveillance marker 10 according to the present invention, is a band in the shape of a flat ring and is made of sheet metal having special magnetic properties, namely, it has high magnetic permeability and low magnetic coercivity and is easily magnetically saturable. The marker 10 may be made of permalloy or material having magnetic properties similar to permalloy, such as amorphous magnetic material. A suitable material is described in U.S. Patent No. 5,146,204. The marker 10 of Fig. 1 has a plurality of elongated cutouts 12 spaced apart along the length of the band. Each cutout is contained entirely within the band and thereby forms a pair of very thin narrowed segments 10a and 10b which are parallel to each other and which interconnect full width segments 10c. These cutouts 12 are preferably evenly distributed around the marker 10. By way of example, the marker 10 has and inner diameter d of 16 millimeters and an outer diameter D of 22 millimeters. The width of the band which forms the ring is thus about three millimeters. Also, by way of example, the marker 10 has a thickness of about 15 microns. The cutouts 12 have a width of about two millimeters and they extend over an arc of about 36°. Thus the thin narrowed segments 10a and 10b each have a width of about 0.5 millimeters and a length of about six millimeters. It has been found that a marker with the above described shape and dimensions and made of permalloy will produce good responses to a magnetic interrogating field in an electronic article surveillance system, with reduced dependence on orientation relative to the interrogating field. These good responses are due firstly, to the fact that the cutouts 12 produce very narrow lengths 10a and 10b in which the ratio of length to cross-sectional area is high, and secondly, to the fact that the regions 10c between the strips provide large masses which will provide regions for deactivation purpose. In addition, because the marker is formed in a closed loop and because the lengths 10a and 10b extend in different directions at different locations along the marker, the orientation sensitivity of the marker relative to the direction of the interrogating magnetic field is minimized. The marker 10 is preferably made from a sheet of high permeability low coercivity readily magnetizable material which is then formed into the above described configuration by means of conventional photoetching techniques. Other techniques, such as mechanical stamping, may also be used. Fig. 2 shows the marker 10 of Fig. 1 as attached to a compact disc 14. As can be seen, the marker fits just around the center opening 14b of the disc 14. Preferably the marker is laminated into the disc during manufacture thereof. In this way the marker cannot be easily removed. The marker may also have laminated thereunder or thereon, segments 16 of a relatively highcoercivity magnetic material which can be magnetized by an authorized person to desensitize the marker so that it will not produce responses upon being carried through a magnetic interrogation field. Figs. 3 and 4 show alternative configurations of the marker of the present invention. As shown in Fig. 3, a marker 20 is formed of a closed loop band in the shape of a square while in Fig. 4 a marker 30 is formed of a closed loop band in the shape of a triangle. In each case the marker is provided with spaced apart cutouts 22 and 32. Each of these cutouts form two thin strips of target material 20a and 20b and 30a and 30b, respectively, which are interspersed with segment 20c and 30c of full width material. As can be seen, the different thin strips in each marker extend in different directions to minimize orientation sensitivity. It will be appreciated that other polygonal marker configurations may be used in accordance with the present invention. It follows from the foregoing, that narrowed regions could be formed by lateral cutouts, which are located on the inner or outer, or both the inner and outer sides of the band.

Claims: An electronic surveillance marker, comprising a band (10) of material extending in a flat plane and being in the shape of a closed loop, said material having magnetic properties of magnetic permeability, coercivity and saturability similar to those of permalloy or amorphous magnetic metal, wherein said marker has multidirectional sensitivity, characterized in that said band (10) has a plurality of cutout regions (12) spaced along the length of said band (10), said cutout regions (12) each being enclosed between the inner and outer edges of said band (10) such that said cutout regions (12) each form a pair of narrow segments (10a, 10b) of said material on opposite sides of each cutout region (12). An electronic surveillance marker according to claim 1, characterized in that there are at least five of said cutout regions (12) distributed along the length of said band (10). An electronic surveillance marker according to claim 1 or 2, characterized in that each of said cutout regions (12) extends a substantial distance along said band (10). An electronic surveillance marker according to any one of claims 1-3, characterized in that said material is an amorphous metallic alloy. An electronic surveillance marker according to any one of claims 1-4, characterized in that said band (10) has a width of about three millimeters and a thickness of about fifteen microns. An electronic surveillance marker according to claim 5, characterized in that said cutout regions (12) have a width of about two millimeters and a length of about six millimeters. An electronic surveillance marker according to any one of claims 1-6, characterized in that each cutout region (12) is centered in said band (10) so as to define said two narrowed segments (10a, 10b) of said material on opposite sides thereof each having a width of about 0,5 millimeters. An electronic surveillance marker according to any one of claims 1-7, chracterized in that said band (10) is in the shape of a ring. An electronic surveillance marker according to any one of claims 1-7, characterized in that said band is in the shape of an polygon (20, 30). An electronic surveillance marker according to claim 9, characterized in that said band is in the shape of a square (20). An electronic surveillance marker according to claim 9, characterized in that said band is in the shape of a triangle (30). In combination, a compact disc record (14) having a center opening and an electronic surveillance marker, wherein said surveillance marker has mutidirectional sensitivity and comprises a band (10) of material in the shape of a ring extending in a flat plane being mounted on said disc (14) in a position surrounding said center opening in said disc (14), said material having magnetic properties of high permeability, low coercivity and being readily magnetically saturable, said combination of said compact disc (14) and said surveillance marker having rotational balance, wherein said band (10) has a plurality of spaced apart cutout regions (12) therealong which form narrowed regions (10a, 10b) along said band (10), said cutout regions (12) being centered in said band (10) so as to define said narrowed regions (10a, 10b) on opposite sides of each cutout region (12).

Inventors: DONALDSON DENNIS P, SOLASKI THOMAS P, ZHOU PETER Y, DONALDSON, DENNIS P., SOLASKI, THOMAS P., ZHOU, PETER Y.
Applicants: SENSORMATIC ELECTRONICS CORP, SENSORMATIC ELECTRONICS CORPORATION
Application year: 1994
Main classification: G08B13
IPC codes: G01V3, G01V15, G08B13, H01F1